MLA

Whitman, Walt, 1819-1892. Millis, William H.. Box 1, folder 29. 1868 Jan 20. https://collections.library.yale.edu/catalog/2053663.

APA, 6th edition

Whitman, Walt, 1819-1892. (["Box 1, folder 29"])(1868 Jan 20)[Millis, William H.]. https://collections.library.yale.edu/catalog/2053663.